Displacement method for preparation of metal borohydrides



United States Patent DISPLACEMENT METHOD FOR PREPARATION OF lVIETAL BOROHYDRIDES George F. Hutr, Fox Chapel, and Albert D. McElroy,

Evans City, 'Pa., assignors to Callery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania 5 N0 Drawing. Filed Apr. 12, 1954, S81. No. 422,646,

7 Claims. (01.23-14) This invention relates to a new and improved method for the preparation of metal borohydrides and more particularly to anew and improved method for preparing a desired metal borohydride in substantial yields from another metal borohydride by displacement of one metal by another metal.

In recent years the borohydrides have received considerable attention because of their importance as selective reduoing agents in chemical synthesis and as a readily available source of hydrogen. Sodium, potassium and lithium borohydrides are the only borohydrides producedon a commercial scale because the other borohydrides are normally diflicult to prepare. Magnesium borohydride is of special interest because of its high hydrogen content per pound.

Several methods have been developed for preparing metal borohydrides one of which is the reaction of a metal alkyl with diborane. Another method consists of the reaction of a metal hydride with methyl borate while still another method utilizes a metathetic reaction between a metal borohydride and a metal halide. These methods have disadvantages such as the use of starting materials which are either expensive or not readily available, relatively low yields or complete inoperativeness of some of these methods in the preparation of certain borohydrides.

It is one of the objects of this invention to provide a new and improved method for the preparation of metal borohydrides in substantial yields from readily available borohydrides such as sodium, potassium or lithium borohydride.

Another object of this invention is to provide an improved method for preparing metal borohydrides from sodium borohydride by displacement of sodium by another metal.

A further object is to provide an improved method for preparing metal borohydrides in which sodium borohydride is dissolved in an ionizing non-aqueous solvent and the resulting solution is placed in contact with mercury and the metal of the borohydride desired.

Other objects of this invention Will become apparent from time to time throughout the specification and claims as hereinafter related.

This new and improved process will be described more fully in the specification and the novelty thereof Will be particularly pointed out and distinctly claimed.

In this invention a solution of an ionic metal borohydride in an ionizing non-aqueous solvent is placed in contact with mercury and the desired metal. The mixture is agitated to insure a reasonably rapid reaction and the desired metal displaces the metal ions from the dissolved borohydride. The displaced metal ions form an amalgam with the mercury. If the desired borohydride is insoluble it may be separated by any suitable means. If the desired borohydride is soluble it may be recovered by evaporation of the solvent.

This invention applies to the conversion of one borohydride into another borohydride by means of the general reaction: M-i-M BH +Hg- MBH +M (Hg) where M BH represents a metal borohydride which is readily available and M represents a metal which does not form an amalgam as readily as does M and stands higher in the 5 electromotive series than an M amalgam. To illustrate this invention more completely consider the effect of mercury and diflerent solvents on the electromotive series of the metals. The ordinary order of the electromotive series of the more active metals in water is Li, Rb, K, Na, Sr, Ba, Ca, Mg, Al. When these metals are arranged in the electromotive order of their amalgams we find that their order in water is Li, Mg, Ca, Sr, K, Na, Rb, Ba. When these metals are arranged in the electromotive order of their amalgams in liquid NH we find the order changed to Li, Ca, Sr, Na, K. It is thus apparent that by a proper selection of the solvent in which a replacement reaction is to be carried out and by the use of mercury to amalgamate preferentially with the replaced metal the above general reaction may be carried out even where metal M is normally considered to be less active than metal M the mercury pool and the magnesium metal were connected by an external electrical circuit through a galvanometer, the deflection of the galvanometer indicated that the direction of positive current in the external circuit was from the mercury to the magnesium. This observation proved that the reaction is a spontaneous reaction.

In another experiment a solution of 3 g. of sodium borohydride in isopropyl amine was allowed to stand over magnesium chips on 600 g. of mercury for several days. The magnesium borohydride deposited as a White crystalline material in the reaction vessel. A yield of percent was obtained. If the magnesium is finely divided or dispersed in the solvent in a colloidal state the reaction proceeds more rapidly. When liquid ammonia is used as the solvent, the product will be the hexammoniate of magnesium borohydride [Mg(BH -6NH which can be converted to the diammoniate by heating at C. More complete removal of the ammonia may be etfected by evacuation at higher temperatures or by reacting the ammoniate With a material which will complex preferentially with ammonia.

In another experiment, calcium borohydride was prepared by dissolving 5 g. of sodium borohydride in ml. of liquid ammonia in a flask containing 2000 g. of mercury. A stoichiometric quantity of calcium was dissolved in this solution and calcium borohydride was recovered as the product of reaction. The yield was low due to the formation of sodium and calcium amide, which can be avoided by reacting calcium amalgam directly with the sodium borohydride.

In another experiment magnesium borohydride was prepared as follows: six grams of sodium borohydride was dissolved in triethylene glycol dimethyl ether eral days. A 25 percent conversion to Mg(BH was effected. In another experiment dimethoxyethane was as the solvent and a 15 percent conversion to mag-,

nesium borohydride resulted.

"From the foregoing examples it is to be understood that i the process described can be used to prepare borohydrides of metals according to the reaction v subject to the conditions of relative activity of the metals g which the metal used is soluble or readily forms a colloidal dispersion. Other experiments show that other organic amines such as methylamine, dimethylamine, ethyl amine, pyridine, and higher'ethers such as the dialkyl ethers of polyethylene glycol, are satisfactory for this general reaction. 'Whilewe have described a few embodiments of our invention it is to be understood that within the scope of the claims appended hereto this invention may be practiced otherwise than as specifically described. Having'thus described our invention what we desire to claim and secure by Letters Patent of the U nited States is: V

l. A method of preparing metal borohydrides comprise ing reacting a solution of an ionic metal'borohydride selected from the group consisting of sodium, potassium and lithium borohydrides in a non-aqueous ionizing solvent selected from the group consisting of polyethylene glycol di-lower alkyl ethers, lower alkyl amines, pyridine and liquid ammonia with a metal of the desired borohydride in the presence of a substantial quantity of mercury, the metal of the desired borohydride forming an -4 less readily than the metal of said dissolved ionic borohydride, and recovering the resulting metal borohydride thus formed. o

2. A method of preparing a metal borohydride of the group consisting of magnesium and calcium borohydrides which comprises reacting a solution of sodium borohydride in a non-aqueous solvent of the group consisting of polyethylene glycol di-lower alkyl ethers, lower alkyl amines, pyridine and liquidammonia with a metaLselected from '-the class consisting ofcaloium and magnesium; in the presence of mercurytand recovering the re,- sulting borohydride of the group consisting of and calcium borohydridesformed. V =11 3. A method of preparing magnesium borohydride comprising reacting a solution of sodium borohydride in isopropylamine with magnesium in the presence of mercury and recovering the magnesium borohydride formed.

4. A method of preparing magnesium borohydride comprising reacting a solution of sodium borohydride intetraethyleneglycol dimethyl ether with magnesium in the presence of mercury and recovering the magnesium boro' hydride formed. a

5. A method of preparing magnesium borohydridelooinprising reacting a solution of sodium borohydride in triethyleneglycol dimethyl ether with magnesium amalgam and recovering the magnesium borohydride formed.

6. A'method of preparingcalcium borohydride comprising reacting a solution of sodium borohydride in liquid ammonia with calcium in the presence of mercury and recovering the calcium borohydride formed.

7. A method according to claim 6 in which the calcium is introduced as'calcium amalgam.

References Cited in the file of this patent Mellor: Modern Inorganic Chemistry, pages 208-9 (1951), published by Long'mans, Green and Co., London. 

1. A METHOD OF PREPARING METAL BOROHYDRIDES COMPRISING REACTING A SOLUTION OF AN IONIC METAL BOROHYDRIDE SELECTED FROM THE GROUP CONSISTING OF SODIU, POTASSIUM AND LITHIUM BOROHYDRIDES IN A NON-AQUEOUS IONIZING SOLVENT SELECTED FROM THE GROUP CONSISTING OF POLYETHYLENE GLYCOL DI-LOWER ALKYL ETHERS, LOWER ALKYL AMINES, PYRIDINE AND LIQUID AMMONIA WITH A METAL OF THE DESIRED BOROHYDRIDE IN THE PRESENCE OF A SUBSTANTIAL QUANTITY OF MERCURY, THE METAL OF THE DESIRED BOROHYDRIDE FORMING AN AMALGAM LESS READILY THAN THE METAL OF SAID DISSOLVED IONIC BOROHYDRIDE, AND RECOVERING THE RESULTING METAL BOROHYDRIGE THUS FORMED. 